1. Field of the Invention
The present invention relates to a throwaway insert (hereinafter, simply referred to as a insert) which is mounted to a tool body of a throwaway-type rotary cutting tool (hereinafter, simply referred to as a rotary cutting tool) to constitute cutting edges of the rotary cutting tool, and a rotary cutting tool having such a insert mounted thereto.
2. Description of the Related Art
As this type of insert and rotary cutting tool, for example, a cutting insert (insert) comprising at least one cutting edge (a main cutting edge) defined between a rake face and a relief face of the insert, and a rotary milling machine (a rotary cutting tool) having a cylindrical holder (a tool body) and such a cutting insert are proposed in Japanese Unexamined Patent Application Publication No. 2-298414. In this cutting insert, the rake face and the relief face is continuously curved. As a result, a rake angle and a relief angle which are defined with respect to the cylindrical holder, that is, which are defined with the insert mounted to the tool body, are made invariable along the length of the cutting edge. Further, the inventor of the present invention proposes a insert and a rotary cutting tool having such a insert mounted thereto in Japanese Unexamined Patent Application Publication No. 2003-334716. In this insert, a main cutting edge is formed so as to be connected to one end of a cutting edge of a nose formed at a corner of a rake face. The main cutting edge has a tapered cutting edge connected to one end of the cutting edge of the nose, and a curved cutting edge connected to a joint point at one end of the tapered cutting edge opposite to the nose. The curved cutting edge is formed so as to be included on a predetermined cylindrical face with a rotation axis of a tool body as a center axis in a state in which the insert is mounted to the tool body such that the nose is positioned at the outer side of a distal end of the tool, and the tapered cutting edge is formed to extend inward from the cylindrical face as it approaches the nose from the curved cutting edge.
However, in the insert and rotary cutting tool disclosed in Japanese Unexamined Patent Application Publication No. 2-298414, the cutting edge is curved, and a curved side portion of a plane which intersects a cylinder constituting a cylindrical surface of the cutting edge is formed at an angle corresponding to an axial rake angle of the insert in a milling cutter. In other words, the main cutting edge is formed to be positioned on an intersecting ridgeline of the cylindrical surface of the rotary cutting tool about its rotating axis and a plane which intersects the cylindrical surface so as to form the constant axial rake angle. Thus, the main cutting edge has a shape of a straight line in side view as seen from the direction along the plane so as to form the constant axial rake angle with the insert mounted to the tool body. For this reason, the chips produced by the main cutting edge flows out onto the rake face while entirely abutting it, which makes it difficult to separate chips and cause an increase in cutting resistance. In particular, when a high ductile metal material such as aluminum is intended to be cut at high speed, a portion of the material positioned further outside than the cylindrical surface at a distal end of the main cutting edge (at the nose in Japanese Unexamined Patent Application Publication No. 2003-334716) is plucked off so as to be pulled out by chips which is hard to be separated. As a result, the rotation locus of the main cutting edge does not become a cylindrical surface about the rotation axis of the rotary cutting tool precisely, but the chips tend to be bitten toward the outer peripheral side beyond the cylindrical surface instead, but the verticalness of a vertically machined surface may be deteriorated instead.
However, according to the insert and rotary cutting tool disclosed in the Japanese Unexamined Patent Application Publication No. 2003-334716, the tapered cutting edge is formed to retreat inward from the cylindrical surface including the curved cutting edge as it approaches the nose. Thus, even when a material with such high ductility is cut at high speed, a portion of the material which is positioned at the outer peripheral side of the nose at the distal end of the insert is plucked off so as to be pulled out with the discharge of chips at other portions, thereby suppressing a tendency that the distal end of the insert bites into the workpiece toward the outer peripheral side. As a result, it is possible to cut a vertically machined surface with high surface quality. Further, Japanese Unexamined Patent Application Publication No. 2003-334716 discloses a construction in which the curved cutting edge is formed such that the distance of the curved cutting edge from the bottom face (mounting face) of the insert gradually decreases as it goes away from the nose while protruding forward in the rotating direction of the tool body. As a result, it is possible to satisfactorily separate chips even for the high ductile metal as described above and thereby to perform high-speed cutting.
Meanwhile, as disclosed in Japanese Unexamined Patent Application Publication Nos. 2-298414 and 2003-334716, in the insert in which the main cutting edge and the curved cutting edge of the main cutting edge is constructed to be included on the cylindrical surface with the rotation axis of the tool body of the rotary cutting tool as a center axis, if the projection line obtained by projecting the rotation locus of the curved cutting edge about the axis onto the plane including the axis is positioned to be inclined with respect to the cylindrical surface to include the curved cutting edge without precisely including the curved cutting edge on the cylindrical surface, for example, because the insert is not preciously mounted to the tool body, there is a problem in that the precision of a vertically machined surface may deteriorate. In particular, if the curved cutting edge is not positioned so as to be included on the cylindrical surface as such, for example, when the rotary cutting tool is moved toward the distal end in the rotating axis thereof step by step by the length of the cutting edge to perform cutting in forming a vertically machined surface which is deeper than the effective cutting edge length of the main cutting edge, it is possible to give high surface quality to individual vertically machined surfaces to be cut at each stage. However, step difference become conspicuous at a joint between adjacent vertically machined surfaces, which requires a finishing process after cutting. This causes the tool body, for example, to be bent during cutting. This is also true of a case in which the cylindrical surfaces to include the curved cutting edge during cutting at each stage coincides with each other.
Further, in the insert disclosed in Japanese Unexamined Patent Application Publication No. 2003-334716, the curved cutting edge and the tapered cutting edge is connected to each other so that respective tangential lines form a predetermined angle at a joint point between the curved and tapered cutting edges. As a result, chips formed by both the cutting edges are grown in different directions from the joint point as a boundary are easily separated from each other. Therefore, the chips formed by the curved cutting edge can further prevent the material positioned at the outer peripheral side of the nose from being pulled out. On the contrary, if the curved cutting edge is not positioned so as to be preciously included on the cylindrical surface, there is a problem in that the joint between the vertically machined surfaces when the rotary cutting tool is moved step by step in the axial direction to perform cutting tends to be conspicuous. Moreover, the curved cutting edge is formed such that the distance thereof from the bottom face (mounting face) of the insert gradually decreases as the curved cutting edge goes away from the nose while the curved cutting edge protrudes forward in the rotating direction of the tool body. As a result, Japanese Unexamined Patent Application Publication No. 2003-334716 discloses a construction in which chip separation is improved even for the above-described high ductile metal to perform high-speed cutting. However, in order to allow higher speed cutting, further improvement of the chip separation is expected.